1. Field of the Invention
The present invention relates to the production of portland cement. More particularly, the invention relates to raw mixes for producing portland cement clinker, and especially to cement compositions which develop high compressive strengths at early ages.
2. Description of the Prior Art
In the typical commercial production of portland-type cements, a calcareous type material, such as limestone, and an argillaceous type material, such as clay, are used to obtain a mixture of lime, aluminum oxide, silicon dioxide, and ferric oxide. These "raw" materials are first pulverized into a homogeneous mixture, either in dry or slurry form, and then thermally processed in a kiln, usually or a rotary type, at temperatures normally ranging from about 1,400.degree. C. to about 1,600.degree. C. to form solid "clinkers." The basic reaction is such that the lime (usually as CaCO.sub.3), upon heating, releases carbon dioxide (CO.sub.2) to form calcium oxide (CaO) or free lime which in turn reacts with the alumina (Al.sub.2 O.sub.3), iron oxide (Fe.sub.2 O.sub.3), and silicon dioxide (SiO.sub.2) to form the components of cement clinker. Clinkers are in turn ground to form a fine-powdered cement. Certain "mineralizers" may be added to the raw mix prior to the "clinkering," and certain "additions" (such as gypsum) may be added to a clinker during grinding to improve the strength and setting properties of the resulting cement. The composition of the cement depends upon the nature and proportion of the raw materials, mineralizers and additions employed, as well as the temperature of ignition and the extent of grinding.
The major components of portland cement clinker are calcium silicates (C.sub.3 S and C.sub.2 S), tri-calcium aluminate (C.sub.3 A), and calcium alumino-ferrite (C.sub.4 AF), all of which form the hydraulically active ingredients of portland cements. For convenience herein, certain abbreviations common to cement technology for thermally processed materials are used as follows:
C represents calcium oxide (CaO) PA1 A represents aluminum oxide (Al.sub.2 O.sub.3) PA1 F represents ferric oxide (Fe.sub.2 O.sub.3) PA1 S represents silicon dioxide (SiO.sub.2).
The calcium silicates are present in two forms, tricalcium silicate (C.sub.3 S) called "alite" and dicalcium silicate (C.sub.2 S) called "belite," in proportions which depend upon the composition of the raw mix. Such silicates account, in substantial part, for the strength and setting properties of the cement.
Portland cements are classified into five major types by the ASTM, according to chemical composition and physical properties. The American Petroleum Institute (API) has classified cements into at least nine categories, such as designations A through H and J. Similar or equivalent ASTM and API cements will hereinafter be referred to exclusively by ASTM designations.
Type III (ASTM) portland cement is an early compressive strength, quick hardening cement that differs from other types of portland cement in that the alite and aluminate contents, and, in particular the fineness, are generally relatively high. Type III cements are conventionally made by forming clinkers having high alite and aluminate contents and grinding the clinkers to a much higher fineness (surface area) than normal. As a result, production of Type III cements may be appreciably more energy intensive than production of Types I, II or V. Moreover, the use of additives such as calcium chloride, which are commonly used to achieve high early compressive strengths in concrete can be potentially harmful with respect to the long term durability of the resulting concrete.
Cements having high early compressive strength are required in order to satisfy industrial needs of current construction practices. For example, in construction use, where concrete is cast into forms, the forms are not normally available for reuse for one and often several days while the cement develops sufficient strength for removal of the form. Cements having high early compressive strengths allow early removal and reuse of the forms, and can also reduce the necessary curing time. Such cements are also highly desirable for prestressed and pretension applications as well as in the construction and repair of highways.
Previous attempts to formulate such cements by modifying clinker reactivity have achieved only limited success. More often, Type III portland cements are produced by grinding high C.sub.3 S, high C.sub.3 A clinkers to a very high fineness. However, due to the high cost of the electrical energy required for fine grinding, more economically produced cement compositions of improved early compressive strength are still being sought, particularly with respect to Type III portland cement compositions.
Accordingly, an object of the invention is to produce a cement which upon hydration will develop a high early compressive strength.
Another object of the present invention is to provide a cement having a high early compressive strength without the necessity of extensive grinding during its preparation.
A further object still is to provide an inexpensive method for manufacturing modified portland-type cements from a mixture of raw materials.
Another object of the present invention is to provide a method for manufacturing a high early strength cement with a minimum change in current manufacturing methods.
A still further object of the present invention is to provide a method for manufacturing cement using presently available equipment.
These and other objects and advantages of the invention will become apparent from the following description.